For the unit type mappings, please see the Unit Types documentation.
Additionally, you can choose to exclude metadata from the units by specifying includeMetadata=false in your query parameters.

Note: Due to the possible amount of data returned, the maximum limit
size for this API call is 100.
If you specify a limit above this size, you will receive an error with the code 400: bad request.

Note: under the Twitter Developer Policy,
and the Twitter Terms of Service,
we are not allowed to export the content of tweets gathered via our API. When a unit is returned that
contains data from Twitter, the unit text and title will be empty, however, a new fields for twitterId and
tweetUrl may be included.
The tweet can be then gotten via the Twitter API using the Twitter GET
statuses/lookup
API call.